Recently I made a stereo FM transmitter, from a kit, to broadcast from my PC to any FM radio in the house.
If I tune into the broadcast frequency with the computer turned off, there is a nice clear sound of nothing and I have to turn the volume up a fair way before the noise starts to come in - good sign.
Now when I turn the computer on to play an mp3 there is a lot of noise. I used a good, well shielded microphone cable and shielded jack to connect to the soundcard, but this has not fixed the problem. I even wrapped the transmitter box in grounded foil without effect.
Can I get rid of this noise so that I can listen to my mp3 of “A Walk in the Black Forest” without any hiss.
Check for ground and wiring faults. The computer’s case should be providing adequate EMI shielding in both directions. Also, see if turning off the monitor helps.
Unplugging the transmitter from the PC removes the static, so maybe it is something to do with grounding of the card rather than the transmitter?? When I turn the volume up on my stereo tuned into the transmitter it almost reached 11 before I get much noise.
How is the transmitter powered? Does the power come from the computer or from an external power supply? Have you tried going from a portable CD player or something instead of the computer to verify that all of the noise is coming from the computer (I suspect it is, but it doesn’t hurt to be sure)? If it’s powered from the computer you’ll probably need to add quite a bit of power supply filtering. Computers use switching supplies, which are noisy little buggers.
You can try moving your sound card over into a different slot. This could move the sound card farther from the source of the interference.
A ferrite collar on the cable might help.
A low pass filter set for say 20 KHz on the input might help quite a bit too. A lower input impedence would probably help too, if you are going to be mucking with the input circuitry.
Do all of the integrated circuits in your gizmo have bypass caps located very close to the chip? Add some if they don’t.
How is the transmitter grounded (to earth ground)?
The power comes from 2 x 1.5v cells (for the moment).
I tried the ferrite collar - no improvement.
The transmitter is grounded to the sheilding of the cable, which I presume is grounded to the computer chassis via jack.
It looks like (from my limited understanding) from the circuit diagram, that the chip has bypass filters close by.
Last night I moved the transmitter to another computer and it worked perfectly. This is an old machine with a sound card.
The machine I first used is much newer, but does not have a sound card. It has an inbuilt soundcard in the motherboard. So must be something to do with the lack of a soundcard (maybe).
I will put a sound card in my new machine and see if that fixes it.